As functional devices used in electric and electronic applications and optical applications, there are known electroluminescent devices capable of emitting light by applying an electric voltage thereto. The electroluminescent devices are generally classified into inorganic electroluminescent devices having a light-emitting layer formed of an inorganic electroluminescent material, and organic electroluminescent devices having a light-emitting layer formed of an organic electroluminescent material.
In particular, the organic electroluminescent devices using a thin film material which is excited and emits light by flowing an electric current therethrough have a large latent possibility of use in extensive applications including displays for cellular phones, personal digital assistants (PDA), displays for computers, information displays for automobiles, TV monitors and general illumination apparatuses, because they are capable of emitting light with a high luminance even by applying a low electric voltage thereto.
On the other hand, the inorganic electroluminescent materials are effective as an illumination element requiring a good flexibility because they are formed into devices on a paper or a polymer film by printing techniques. In addition, the inorganic electroluminescent materials are less capable of emitting light with a high luminance as compared to the organic electroluminescent materials, but have advantages such as excellent long-term stability and stable light emission even under severe conditions such as high-temperature conditions. For these reasons, the inorganic electroluminescent devices having a light-emitting layer formed of the inorganic electroluminescent materials have been studied and researched for utilization in the application fields for which a good weather resistance, a good heat resistance, a good long-term stability, etc., are needed.
As one of the electroluminescent devices using the inorganic electroluminescent materials, there are known electroluminescent sheets. The electroluminescent sheets usually have such a structure in which a substrate, an electrode and an electroluminescent layer are successively laminated. The electroluminescent layer in the electroluminescent sheets is formed of the inorganic electroluminescent materials.
The electroluminescent sheets are used, for example, as a back light for advertizing media and decorating media which are disposed on windows of commercial buildings or automobiles, or security sheets, etc., because they are excellent in weather resistance, heat resistance and long-term stability.
For example, Patent Document 1 discloses and proposes an advertizing method using a sign board in which images indicating an object to be advertized are provided on a surface of a film-shaped illuminant, and an advertizing sign board in which images indicating an object to be advertized are provided on a surface of a film-shaped illuminant. In Patent Document 1, it is described that the film-shaped illuminant is constructed from an electroluminescent device having a pair of electrode layers and an electroluminescent layer interposed between the electrode layers which is capable of emitting light by applying an electric field thereto. The electroluminescent layer is made of an inorganic electroluminescent material such as zinc sulfide and zinc oxide. In addition, in Patent Document 2, there is disclosed an electroluminescent lamp in which a light-emitting layer, a dielectric layer and a back electrode are successively laminated on a transparent electrode formed on a transparent film made of polyethylene terephthalate, etc.
Meanwhile, with recent high consciousness of ensuring the privacy, studies have been made on the application of the electroluminescent sheets to members for privacy protection which are to be fitted to windows of vehicles or buildings such as commercial buildings and houses. Therefore, the light transmittance (transparency) of the electroluminescent sheets becomes important as one of properties required therefor. However, the light-emitting members as disclosed in Patent Documents 1 and 2 fail to exhibit a good light transmittance so that a back side of these members is invisible and unrecognizable, for example, when attached onto a transparent substrate such as a window glass.
The electroluminescent layer made of the inorganic electroluminescent material is generally formed by a sintering method, a laser abrasion method, a molecular beam epitaxy (MBE) method, a physical vapor deposition (PVD) such as a sputtering method and a vacuum deposition method, or a chemical vapor deposition (CVD) method (for example, refer to Patent Document 3). However, since these methods need complicated procedures or use of expensive apparatuses, there tends to occur such a problem that the electroluminescent devices obtained by the methods inevitably become very expensive.
To solve the above problem, there is disclosed, for example, an electroluminescent lamp having such a layer structure in which an electroluminescent layer, a dielectric layer and a back electrode are successively laminated on a transparent electrode formed on a transparent film by a screen printing method, etc., using coating materials prepared by respectively dispersing an inorganic electroluminescent material obtained by activating zinc sulfide with copper and a high-dielectric material such as barium titanate in an organic binder (for example, refer to Patent Document 4).
The above technique described in Patent Document 4 may be performed without need of expensive apparatuses and complicated operations as compared to the technique described in Patent Document 3, but requires a number of steps and is unsuitable for mass production of the aimed product.
Patent Document 1: JP-A 2003-15557
Patent Document 2: JP-B 7-58636
Patent Document 3: JP-A 2005-290068
Patent Document 4: JP-A 4-190586